Along with growth of high-speed and high capacity radio communications, it is expected that multi-branch antenna architecture such as active antenna systems (AASs) or massive MIMO (multi input multiple output) systems will become mainstream in the days ahead. In multi-branch antenna architecture, individual radio frequency (RF) transmitting circuits and RF receiving circuits are connected to respective antenna elements. An antenna port can be formed by a single antenna elements or a group of antenna elements.
When open-loop MIMO communication is carried out using multi-branch antenna systems, calibration is performed to reduce phase differences between antenna branches. If calibration is inappropriate, directivity diversity control is adversely affected, and base station coverage changes. The phase of signals between branches also changes due to characteristics of the RF circuits. For these reasons, it is desired to perform calibration as needed.
In the conventional technique, a known reference signal is typically used for calibration. On the other hand, a technique for generating a special signal for calibration is proposed. See, for example, Japanese Patent Laid-open Publication No. 2005-348235 A. In this proposed technique, an actually used downlink signal is employed as a reference signal. Antenna branches are switched in turn to monitor the downlink transmission signal and calibration is performed every time the output level of the amplifier of the RF transmission circuit varies.